Dental crown and bridge restorations are often made with a metal base having a jacket or covering of dental porcelain so that the restoration will closely resemble the natural tooth. Such restorations have been used for many years.
The type of porcelain that is most often employed today in such restorations is that described in the two Weinstein et al. patents, U.S. Pat. Nos. 3,052,982 and 3,052,983. The Weinstein et al. patents address the problem of preparing a porcelain whose coefficient of thermal expansion will match that of the metal base so that excessive stress formation will not occur during the production of the restoration.
The solution proposed by Weinstein et al. is to make a dental porcelain composed of two different frits, one having a high coefficient of expansion and the other having a much lower coefficient of expansion, to result in a porcelain having a coefficient of expansion intermediate between the two materials, and which will match the dental alloy employed as the base.
If one attempts to make a single frit ceramic material whose composition is the average of the two components employed in the two-frit mixture of the type contemplated by Weinstein et al., the resulting porcelain will have too low a coefficient of expansion, and cannot, therefore, be employed with metal-base dental restorations.
The present invention is based upon the discovery of a single frit glass-ceramic material that has a coefficient of expansion that will match that of the majority of metal alloys employed in producing dental restorations. The use of a single frit glass ceramic material reduces the criticality in mixing the components so as to obtain the desired coefficient of thermal expansion and fusion temperature. As a result, better control over the desired properties and/or composition can be obtained.